Respect --override flag for affinity predictions; use all devices by passing --devices=0

[timodonnell]

A few small things:

1. Fixed an issue where affinity predictions were being re-used (not rerun) even when --override is passed
2. Added support for passing --devices=0 to use all available devices. Left the default unchanged at 1 for now.
3. Cleaned up the logic around downsizing the number of devices when there are fewer predictions than requested devices. Before it ignored the number of affinity predictions to generate. It also had an issue where when it reduced us to 1 device, we could still end up using DDPStrategy since the strategy decision happened before the downsizing